This invention relates to the preparation of metallic materials, which may be finely divided and highly porous, from easily reducible salts or from metal salts admixed with a reducing solvent.
In many applications, such as specialty metal strip mill powder rolling processes, metallic coatings, catalysts, conductive inks, and in the production of printed circuit boards, it is desirable to use finely divided metal powders. Moreover, it is highly desirable that the particles have a size and morphology which encourage intimate metal bonding.
Many methods for obtaining metals in powder form are known. Among the known methods, processes involving atomization of molten metal and mechanical grinding or milling predominate. Atomization processes, including variations on the basic concept, are disclosed, for example, in U.S. Pat. Nos. 3,325,277, 3,598,567, 3,646,177, 3,764,295, and 3,813,196. An example of the process employing milling or grinding is disclosed in German No. 2,555,131.
Additionally, the physical properties of powders have been manipulated by employing different known processes. For example, powders are produced in the form of agglomerates of solid particles, spherical particles and flakes. See U.S. Pat. Nos. 2,825,108, 3,813,196, and 3,325,277, and German No. 2,555,131.
Recently, a number of more exotic methods such as plasma processes and laser-assisted processes have been reported for producing ultra-fine metallic, nonmetallic and ceramic powders. See, for example, Murarka, Refractory Silicides for VSLI Production, Academic Press, 1983, pp. 115-31, and Danforth et al., "Synthesis of Ceramic Powders by Laser Driven Reactions," Industrial Liaison Program Report No. 10-17-82, ILP Publications Office, M.I.T., Cambridge, Mass. Furthermore, a unique class of very fine and porous ceramic materials has been prepared by a process which requires removal of solvent from a wet gel containing a ceramic powder product at a temperature above the critical temperature of the solvent. This unique class of materials has been given the name "aerogel."
Aerogels are usually produced by dissolving or suspending a metal ion (generally referred to as solute) usually in the form of a metal salt (such as an hydroxide, alkoxide or acetate) in an aqueous or alcohol medium (or both), and venting the solvent under hypercritical conditions. The medium functions to hydrolyze the metal salt to produce a gel comprising the ceramic product and solvent. Upon removal of the solvent as indicated above, a porous, very fine ceramic product can be recovered. A detailed description of this method is reported by S. J. Teichner et al, "Inorganic Oxide Aerogels", Advances in Colloid and Interface Science, Volume 5, 1976, pp. 245-73.